There are many kinds of surface acoustic wave devices such as surface acoustic wave amplifier, convolver, correlator, etc. which utilize non-linearity of a piezoelectric material such as single crystal piezoelectric material, piezoelectric film, piezoelectric ceramic, etc. or non-linearity of increase and decrease of a depletion layer in a semiconductor combined with a piezoelectric film.
FIGS. 1 and 2 are cross-sectional views, each illustrating a prior art structure of such a surface acoustic wave device. Reference numeral 1 refers to a piezoelectric substrate comprising a piezoelectric material of single crystal type, piezoelectric ceramic, or the like, reference numerals 2 and 3 refers to a pair of transducers which have an interdigital configuration, and 4 refers to a center electrode disposed in the center between the transducers 2 and 3 and including a bottom surface electrode 5. Reference numeral 6 refers to a non-piezoelectric substrate comprising a semiconductor, for example, and 7 refers to a piezoelectric film provided on the substrate 6.
With this arrangement, incident surface acoustic waves 9 from the transducers 2 and 3 to the center electrode 4 are partly reflected by the center electrode 4 and become reflected waves 8, as shown in FIG. 3. This invites waveform distortions of output signals from the device.
Such waveform distortions to output signals or deteriorations of surface wave processing characteristics of the device is also often caused by direct waves which have failed to be converted to surface acoustic waves by the transducers 2 and 3 and travel to the center electrode 4 as electromagnetic waves.